1. Field of the Invention
This invention relates to a fine structure body adapted to utilization for Raman spectroscopy. This invention also relates to a process for producing the fine structure body. This invention further relates to a Raman spectroscopic method and apparatus utilizing the fine structure body.
2. Description of the Related Art
Raman spectroscopy is a technique, wherein scattered light, which is obtained from irradiation of light having a single wavelength to a sample substance, is separated into spectral components of the scattered light, wherein a spectrum (hereinbelow referred to as the Raman spectrum) is thereby obtained, and wherein light (hereinbelow referred to as the Raman scattered light) having wavelengths different from the wavelength of the incident light is detected in accordance with the Raman spectrum. The Raman scattered light has a markedly low intensity. Therefore, ordinarily, it is not always possible to detect the Raman scattered light. However, it has been reported that, in cases where a sample substance is adsorbed onto a metal surface, and light is irradiated to the sample substance, the intensity of the Raman scattered light is capable of being enhanced by a factor of approximately 1E+04 to approximately 1E+06. Particularly, it has been known that, with a structure, in which fine metal particles of a nano order have been distributed and located on a surface for adsorption of a sample substance, the Raman scattered light is capable of being enhanced markedly. (Reference may be made to, for example, “A complementary study of surface-enhanced Raman scattering and metal nanorod arrays”, J. L. Yao et al., Pure Appl. Chem., Vol. 72, No. 1, pp. 221-228, 2000.) It has been theorized that the enhancement of the Raman scattered light arises due to localized surface plasmon resonance. Specifically, it has been presumed that free electrons within the fine metal particles undergo resonance with an electric field of light and vibrate, a strong electric field thus occurs in the vicinity of the fine metal particles, and the Raman scattered light is enhanced by the effects of the strong electric field occurring in the vicinity of the fine metal particles.
With a process disclosed in, for example, “A complementary study of surface-enhanced Raman scattering and metal nanorod arrays”, J. L. Yao et al., Pure Appl. Chem., Vol. 72, No. 1, pp. 221-228, 2000, an alumina layer is formed with anodic oxidation processing performed on aluminum, a metal is filled in fine holes, which are formed naturally in a surface layer of the thus formed alumina layer during the anodic oxidation processing, and a device having a structure, in which fine metal particles have been distributed and located, is thereby produced. Specifically, with the disclosed process, after the metal has been filled in the fine holes, a top region of the alumina layer is removed with etching processing, and head regions of the fine metal particles are thus protruded, such that the Raman scattered light may be enhanced by a strong electric field occurring at pointed ends of the head regions of the fine metal particles. With a process disclosed in, for example, U.S. Patent Application Publication No. 20050105085, an alumina layer is formed with anodic oxidation processing performed on aluminum, a metal is filled in fine holes, which are formed naturally in a surface layer of the thus formed alumina layer during the anodic oxidation processing, electroforming is continued even after the filling of the metal into the fine holes has been completed, and a device having a structure, in which spherical gold nanoparticles have been distributed and located, is thereby produced.
However, with each of the disclosed processes described above, the problems are encountered in that the number of processing steps is not capable of being kept small. Also, with each of the disclosed processes described above, the problems are encountered in that, in order for regularity of the fine holes formed by the anodic oxidation processing to be enhanced, it is necessary for Cr to be added. The addition of Cr is not appropriate from the view point of environmental protection. Further, with each of the disclosed processes described above, the problems are encountered in that, since precise condition setting is required for the production of the device, it is not always possible to produce the device with a high reproducibility.
With a process disclosed in, for example, “Tunable Surface-Enhanced Raman Scattering from Large Gold Nanoparticle Arrays”, A. Wei et al., Chem. Phys. Chem., Vol. 2, No. 12, pp. 743-745, 2001, spherical gold nanoparticles are fixed to a surface of a base plate, and a device having a structure, in which the fine metal particles have been distributed and located, is thereby produced.
In cases where measurement for Raman scattering spectral analysis is to be made, a laser beam having a wavelength falling within a near infrared wavelength region (700 nm to 900 nm) is often utilized as the irradiation light. In such cases, in order for a high degree of the enhancement to be obtained, it is necessary that a surface plasmon absorption band of the gold nanoparticles be matched with the near infrared wavelength region. However, in cases where the spherical gold nanoparticles are utilized, it is necessary for the size of each of the gold nanoparticles to be set to be large, and the quantity of gold used becomes large. Therefore, in such cases, the cost is not capable of being kept low. Also, in the cases of the spherical particles, the area of the region, in which the adjacent particles are closest to each other, is small, and therefore a high degree of the enhancement is not capable of being obtained.
In, for example, “Surface-Enhanced Raman Scattering Studies on Aggregated Gold Nanorods”, B. Nikoobakht and M. A. El-Sayed, J. Phys. Chem. A, Vol. 107, No. 18, pp. 3372-3378, 2003, the matter concerning Raman spectral analysis utilizing gold nanorods is reported. A process disclosed in the literature described above utilizes a two-stage technique, in which the gold nanorods are fixed to silica particles, and in which the gold nanorods having been fixed to the silica particles are thereafter fixed to a surface of a base plate. Also, in the literature described above, a report is made with respect to only the analysis of a surface-active agent, which has been adsorbed to surfaces of the gold nanorods at the time of synthesis of the gold nanorods, and nothing is reported with respect to the surface enhancement by the gold nanorods.
With a process disclosed in, for example, “Fabrication, Characterization, and Application in SERS of Self-Assembled Polyelectrolyte-Gold Nanorod Multilayered Films”, X. Hu et al., J. Phys. Chem. B, Vol. 109, No. 41, pp. 19385-19389, 2005, metal nanorods are fixed by electrostatic attraction to a surface of a glass base plate by use of an alternating adsorption technique, and a device having a structure, in which the metal nanorods have been distributed and located, is thereby produced.
However, with the alternating adsorption technique, the metal nanorods are not capable of being fixed at a high density. Also, with the alternating adsorption technique, the distance between the metal nanorods and the orientation of the metal nanorods are not capable of being controlled. Therefore, it is not always possible to achieve sufficient enhancement of the intensity of the Raman scattered light and reproducible production of the device.
With a process disclosed in, for example, “Surface-Enhanced Non resonance Raman Scattering of Rhodamine 6G Molecules Adsorbed on Gold Nanorod Films”, M. Suzuki et al., Japan. J. Appl. Phys., Vol. 43, No. 4B, pp. L554-L556, 2004, gold nanorods are aggregated at a water phase-oil phase interface, the gold nanorods having been aggregated is then transferred to a surface of a base plate, and a device having a structure, in which the metal nanorods have been distributed and located, is thereby produced.
However, with the technique, in which the gold nanorods are aggregated at the water phase-oil phase interface, the spacing between the gold nanorods is not capable of being controlled, and the gold nanorods are not capable of being distributed and located without defects on the surface of the base plate. Therefore, it is not always possible to achieve sufficient enhancement of the intensity of the Raman scattered light and reproducible production of the device. Also, the technique described above, in which an organic solvent is utilized, is not appropriate from the view point of environmental protection. Further, in, for example, “Surface-Enhanced Non resonance Raman Scattering of Rhodamine 6G Molecules Adsorbed on Gold Nanorod Films”, M. Suzuki et al., Japan. J. Appl. Phys., Vol. 43, No. 4B, pp. L554-L556, 2004, nothing is mentioned with respect to a technique for adjusting the inter-particle distance between the gold nanorods, which inter-particle distance has a large effect on the degree of the enhancement, and a covering rate on the surface of the base plate, which covering rate has a large effect on the degree of the enhancement, and with respect to an optimum value of the inter-particle distance and an optimum value of the covering rate.